Simulating a POST request with a spoofed IP

I'm trying to make a HTTP POST request with a spoofed ip (94.234.170.70 below).

If I make an new HttpWebRequest() and POST it to http://posttestserver.com/post.php (a server for testing POST request) it gets registered. However, using the code below, nothing gets registered.

I compared the two requests in WireShark, and the only difference seems to be that the
HttpWebRequest line gets a green background, while the other once gets black and a
[TCP Retransmission] prefix added to the description in the "Info"-field.

The second requests gets sent using my regular network adapter using your sample code from the "SendingASinglePacketWithSendPacket"-project.